Reviews — Geological Aspects of the Coral-reef Problem. 275 



the origin of these, as well as of reefs with central islands and of 

 raised reefs of all hinds. Professor Davis concludes dennitelj' that 

 both these features are inconsistent with the formation of reefs 

 during still-stand conditions or elevation, and that all the evidence 

 is strongly confirmatory of Darwin's original view of reef-building 

 during subsidence or submergence. 



Furthermore, a distinction must be drawn between the two effects 

 just mentioned: the drowning of an island, with embayment of the 

 coast, maybe brought about by a relative rise of the sea-level brought 

 about by elevation of the sea-floor elsewhere, or by the setting free of 

 water from an ice-sheet, as elaborated by Daly in the theory commonly 

 known as "glacial control". Professor Davis does not consider, 

 however, that either of these processes have played a large part in 

 the production of coral-reefs; he believes that actual subsidence, 

 in many cases of a local nature, is the dominant factor, while the 

 raised reefs must obviously owe their present position to uplift, 

 which also is frequently limited to comparatively small areas, and 

 is sometimes referable to tilting of island groups. 



One of the most significant structural features of coral reefs is the 

 usually well-marked "unconformable" contact of reef and lime- 

 stones with their foundations, the word " unconformable " being here 

 used to mean inclined, while " conformable " means that the base is 

 horizontal. This feature has been neglected by the great majority 

 of writers, many of whom were not geologists. A careful examina- 

 tion of many such contacts, where visible in raised reefs, has shown 

 in nearly all cases pronounced erosion of the underlying, usually 

 volcanic rock, amounting in some cases to 600 or 800 feet. This 

 sloping contact with the eroded surface below is inconsistent with 

 formation of reefs on wave-worn submarine platforms, according to 

 the llein-Murray theory. Even the reefs around the granitic islands 

 on the Seychelles bank may be assumed to possess unconformable 

 contacts, since they clearly rest on sloping granitic spurs, indicating 

 deep erosion and embayed coastlines. It follows, therefore, that 

 here also there must have been submergence. 



The general conclusion reached by Professor Davis is that in the 

 great majority of cases the features exhibited by fringing and barrier 

 reefs can best be explained on the hypothesis of their formation 

 during subsidence. The downward movement is not necessarily 

 continuous ; in fact, it is clear in many cases that it was intermittent 

 and occasionally reversed, and a wide field is thus allowed for local . 

 variations. The problem of atolls now at sea-level is on a somewhat 

 different footing, since the facts are less accessible to observation, 

 but by analogy the same line of reasoning can be applied. The 

 recent work of Foye on the Fiji group is confirmatory of the general 

 ideas here put forward, and this most important paper must be 

 regarded as a step forward in the rehabilitation of Darwin's theory, 

 which has of late years been more or less under a cloud, but is now 

 apparently destined once more to hold the field as the orthodox 

 solution of this difficult but most interesting geological problem. 



